1. Field of the Invention
This invention relates generally to information signal recording and reproducing apparatus and, more particularly, is directed to a noise reduction circuit for reducing noise generally accompanying a reproduced information signal in an information signal recording and reproducing apparatus.
2. Description of the Prior Art
Compandor type noise reduction circuits for reducing noise and distortion which accompany a reproduced information signal are well-known in the art. Such noise reduction circuits are designed to increase the dynamic range of the signal that can be recorded and reproduced from a recording medium such as a magnetic tape. As an example, noise reduction circuits which increase the dynamic range of the signal by approximately 10 dB in the high frequency region by means of level compression and a complementary level expansion of the high frequency components of the input signal supplied thereto, are disclosed in the U.S. Pat. No. 3,631,365 to Dolby and U.S. Pat. No. 3,911,371 to Nakamura et al., the latter having a common assignee herewith.
Generally, these noise reduction circuits use a variable filter circuit which includes a variable resistance element constituted, for example, by an FET or a bipolar junction transistor, to effect the aforementioned compression and expansion operations. However, the use of a variable filter circuit imposes restrictions upon the design of the noise reduction circuit. For example, because of the discrete elements used with the aforementioned transistors, it becomes difficult, if not impossible, to design such variable filter circuits as integrated circuits. Also, the aforementioned transistors used in the variable filter circuits have a temperature dependency which, in many instances, is difficult to control and which may, in fact, cause a direct current shift due to, for example, the resultant changing resistance of the transistor with temperature. This may result in changes or fluctuations in the frequency characteristic of the circuit. Further, the above noise reduction circuits are not interchangeable, that is, an information signal recorded with the Dolby circuit must be reproduced only with the Dolby circuit. In addition to the above restrictions on circuit design, it is difficult to obtain a variable resistance element having precise characteristics. This means that it becomes difficult to improve the desirable characteristics of the noise reduction circuit, whereby the dynamic range thereof can only be increased by approximately 10 to 20 dB.